Non-Isotropic Acoustic Cable

ABSTRACT

Embodiments of the present invention address aliasing problems by providing a plurality of discrete acoustic sensors along a cable whereby acoustic signals may be measured in situations where the fibre optic cable has not been secured to a structure or area by a series of clamps. Acoustic sampling points are achieved by selectively enhancing the acoustic coupling between the outer layer and the at least one optical fibre arrangement, such that acoustic energy may be transmitted selectively from the outer layer to the at least one optical fibre arrangement. The resulting regions of acoustic coupling along the cable allow the optical fibre to detect acoustic signals. Regions between the outer layer and the at least one optical fibre arrangement that contain material which is acoustically insulating further this enhancement since acoustic waves are unable to travel through such mediums, or at least travel through such mediums at a reduced rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/021,319, filed Mar. 11, 2016, which claims priority under 35. U.S.C.§371 to Patent Cooperation Treaty Application No. PCT/GB2014/052679,filed Sep. 4, 2014, which claims priority to GB Application No.1316364.7, filed Sep. 13, 2013, and to GB Application No. 1316362.1,filed Sep. 13, 2013, the entire contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a fibre optic cable, and in someembodiments provides a fibre optic cable which has an acousticsensitivity that is non-isotropic. In other embodiments there isprovided a fibre optic cable which comprises an array of discreteacoustic coupling regions.

Background to the Invention and Prior Art

To detect an acoustic signal, distributed acoustic sensing is commonlyand effectively used. This method employs fibre optic cables to providedistributed acoustic sensing whereby the fibre optic cable acts as astring of discrete acoustic sensors, and an optoelectronic devicemeasures and processes the returning signal. The operations of such adevice is described next.

A pulse of light is sent into the optical fibre, and a small amount oflight is naturally back scattered, along the length of the fibre byRayleigh, Brilliouin and Raman scattering mechanisms. The scatteredlight is captured by the fibre and carried back towards the source wherethe returning signal is measured against time, allowing measurements inthe amplitude, frequency and phase of the scattered light to bedetermined. If an acoustic wave is incident upon the cable, the glassstructure of the optical fibre is caused to contract and expand withinthe vibro-acoustic field, consequently varying the optical path lengthsbetween the back scattered light scattered from different locationsalong the fibre. This variation in path length is measured as a relativephase change, allowing the optical phase angle data to be used tomeasure the position of the acoustic signal at the point at which lightis reflected. The returning signal can also be processed in order todetermine the frequency of oscillation of vibration in the structure.

In known distributed acoustic sensing systems (DAS), standard fibreoptic cables are utilised to obtain a measurement profile from along theentire length of the fibre at intervals ranging from 1-10 metres.Further details regarding the operation of a suitable DAS system, suchas the iDAS™, available from Silixa Limited, of Elstree, UK are given inWO2010/0136809. Systems such as these are able to digitally recordacoustic fields at every interval location along an optical fibre atfrequencies up to 100 kHz. Since the position of the acoustic sensors isknown (the fibre deployment being known), the source of any acousticsignal can be thus identified by means of time-of-arrival calculations.In a typical deployment, the sensing points are usually created byclamps which are used to secure the fibre optic cable to the structureor area it is monitoring.

By way of example, FIG. 1 shows a common arrangement of a known fibreoptic cable 1, comprising at least one optical fibre, contained in aseries of concentric tubular structures. The cable generally comprisesfirstly an inner tubular structure, typically called afibre-in-metal-tube (FIMT) 2, which provides a way of encapsulating verylong lengths of optical fibres 5 within a hermetically sealed tube 4. Ageneral construction of a FIMT 2 includes at least one optical fibre 5encapsulated in a metal tube 4. Additionally, it is common to fill thismetal tube 4 with a thixotropic gel 6 in order to protect the opticalfibres 5 from environmental disturbances, prevent damage frommicro-bending conditions and to help minimise the forces applied duringspooling and deployment of the cable. Most importantly for distributedacoustic sensing, the thixotropic gel 6 supports the optical fibre 5,preventing excessive movement within the metal tube 4 which reduces theamount of resonant frequencies. The FIMT 2 is typically thenencapsulated by a further outer tube 3, usually containing a fillermaterial.

The optical fibres 5 are typically made of flexible, transparent fibresof glass. The filler material 3 surrounding the FIMT 2 has a lowerrefractive index than the optical fibres 5 such that light which hasbeen focused into the optical fibres 5 is confined due to total internalreflection, hence enabling the light to propagate down the length of theoptical fibres 5 without any light being lost.

There are many applications for which distributed acoustic sensing maybe used, for example, monitoring hydraulic fracturing of oil or gasstructures and surveillance methods of assets such as oil or gaspipelines and airport runways. In order to monitor such assets, thefibre optic cables are usually secured to the structure or area byclamps distributed along the length of the fibre optic cable. By way ofexample, FIG. 2 illustrates how fibre optic cables 1 may be used tomonitor structures or areas using distributed acoustic sensing.

FIG. 2 shows a fibre optic cable 1 being used to monitor a pipeline 7that has been deployed underground 9. The fibre optic cable 1 ispositioned to run parallel alongside the pipeline 7 and is secured by aseries of clamps 8, which are distributed along the length of thepipeline 7. These clamps 8 allow the fibre optic cable 1 to monitor thepipeline 7 through distributed acoustic sensing since the clamps 8themselves act as an array of acoustic coupling regions. The clamps 8transmit any vibrations in the pipeline 7, such that the acoustic energyis transmitted to the optical fibres 5.

The clamps are spaced along the fibre at a distance at least equal to orgreater than the sensing resolution of the distributed acoustic sensing,typically 1-5 metres. This provides discrete sensing points along thefibre matched to the sensing resolution and prevents any anti-aliasingeffects in the detected acoustic signal.

In some deployments, however, it is not possible to secure the cablewith clamps, and instead the cable may be inserted in a concrete trenchor the like running parallel to a pipe, well, or any other structurebeing monitored. In this case there are no discrete sensing points as isprovided by the clamps, and hence the fibre can sense at all pointsalong its length.

As a consequence, due to the sensing resolution of the fibre being lessthan the actual resolution of the points at which acoustic energy isbeing sensed, aliasing effects can occur in the signal, due toundersampling.

Another problem faced when using fibre optic cables in distributedacoustic sensing is that acoustic signals incoming from one directionmay overcome acoustic signals incoming from another direction, making itdifficult for the fibre optic cable to detect the latter. This may proveproblematic for certain applications of distributed acoustic sensing.Consider, by way of example, fibre optic cables used for surveillance ofan asset. Acoustic signals emitted by the asset itself may obscure anyacoustic signals incoming from the surroundings towards the asset.However, in security surveillance, it is the incoming acoustic signalscaused by disturbances in regions surrounding the asset that are ofinterest. Therefore, it would be advantageous if the fibre optic cablewas more acoustically sensitive in the directions corresponding to thesurrounding area such that the ability to detect acoustic signals inthese direction is greater.

SUMMARY OF THE INVENTION

Embodiments of the present invention address the above noted aliasingproblem by providing a plurality of discrete acoustic sampling pointsalong a fibre optic cable whereby acoustic signals may be measured insituations where the fibre optic cable has not been secured to astructure or area by a series of clamps, as described in the prior art.Acoustic sampling points are achieved by selectively enhancing theacoustic coupling between an outer layer and at least one optical fibrearrangement, such that acoustic energy may be transmitted selectivelyfrom the outer layer to the optical fibres. The resulting regions ofacoustic coupling along the fibre optic cable allow the optical fibre todetect acoustic signals.

Further embodiments of this invention address the above noted problemsassociated with interfering acoustic signals by providing directionalacoustic sensing. This is achieved by adapting the acoustic response ofa fibre optic cable so as focus the acoustic sensitivity of the fibreoptic cable directionally along the entire length of the fibre opticcable, such that the acoustic sensitivity is non isotropic in the planenormal to the length of the cable. In some embodiments, the adapting ofthe acoustic response is performed by the provision of an acousticallyreactive mass, such as a sleeve or coating, being placed around thefibre optic cable, the mass having spatial acoustic properties requiredto give the directional acoustic response required.

According to one aspect of the present invention, there is provided afibre optic cable, comprising at least one optical fibre arrangement,and at least one outer layer encapsulating the at least one opticalfibre arrangement. The fibre optic cable further comprises an acousticinsulating layer between the at least one optical fibre arrangement andthe outer layer, the insulating layer being interspersed along thelength of the fibre with discrete acoustic coupling regions fortransmitting acoustic energy from the outer layer to the at least oneoptical fibre arrangement.

Preferably, the at least one optical fibre arrangement comprises afibre-in-metal-tube (FIMT), as described in the above prior art. This isa standard and widely used arrangement for the cores of fibre opticcables, therefore existing cables which have already been deployed maybe conveniently adapted to incorporate the features of the presentinvention.

In some embodiments, the acoustic insulating layer includes a layer ofair. Air is a material with low acoustic coupling such that iteffectively absorbs acoustic energy and reduces its transmission.Acoustic coupling relates to the resistance of the material's particlesto the mechanical vibrations of an acoustic signal. That is to say,materials that do not resist the mechanical vibrations easily couplewith the mechanical vibrations and have high acoustic couplingproperties. Since air particles provide a large amount of resistance tothe vibrations, air exhibits low acoustic coupling and is considered tobe a good acoustic insulating material, which is also convenient andcost-effective to use.

In a preferred embodiment of the invention, a filler is inserted betweenthe at least one optical fibre arrangement and the outer layer. Thefiller comprises of built up regions interspersed along the length ofthe fibre optic cable, wherein the built up regions of filler providethe discrete acoustic coupling regions.

The built up regions of filler connect the at least one optical fibrearrangement and the outer layer such that acoustic energy can betransmitted between them, therefore enhancing the acoustic couplingbetween the at least one optical fibre arrangement and the outer layer.Therefore, fibre optic cables with built in discrete acoustic couplingregions may be deployed and used to detect acoustic signals without theuse of clamps securing them to the monitored structure or area.

In another preferred embodiment of the invention, at least one layerconcentrically outside the acoustic insulating layer is narrowed atpoints interspersed along the length of the fibre optic cable so as todivide the acoustic insulating layer and provide discrete acousticcoupling regions.

The narrowed points bridge the gap between the at least one opticalfibre arrangement and the outer layer such that acoustic energy can betransmitted between them, therefore enhancing the acoustic couplingbetween the at least one optical fibre arrangement and the outer layer.Therefore, fibre optic cables with built in discrete acoustic couplingregions may be deployed and used to detect acoustic signals without theuse of clamps securing them to the monitored structure or area.

Preferably, the narrowed points are achieved by crimping the fibre opticcable at points interspersed along its length such that the inner faceof the outer layer immediately next to the insulating layer comes intocontact with the layer inwards of the insulating layer towards the atleast one optical fibre arrangement. In doing this, the crimped portionseffectively short-circuit the insulating layer to provide the discreteacoustic coupling regions.

Preferably, the distance between acoustic coupling regions is at least 1metre. This ensures that the sensing resolution of the fibre matches theactual resolution of the points at which acoustic energy is being sensedso as to avoid aliasing effects as a result of undersampling.

In a further embodiment, the size of the discrete acoustic couplingregions along the length of the fibre optic cable is at most 50 cm. Thisis a suitable size value that ensures that the acoustic coupling regionsare sufficiently small that they provide discrete points to detectacoustic signal, but large enough that they are able to couple withacoustic vibrations.

In another preferred embodiment, the size of the discrete acousticcoupling regions along the length of the fibre optic cable is at least10 cm. This is a preferred size value that ensures that the acousticcoupling regions are sufficiently small that they provide discretepoints to detect acoustic signal, but large enough that they are able tocouple with acoustic vibrations.

According to a further embodiment of the present invention, wherein thediscrete acoustic coupling regions comprise a periodic structure.Preferably, the periodic structure is achieved by dividing the discreteacoustic coupling regions into equal portions. This periodic structureprovides discrete acoustic coupling points within the discrete couplingregion.

Preferably, the size of the equal portions along the length of the fibreoptic cable is at most 5 cm, and the size of the equal portions alongthe length of the fibre optic cable is at least 1 cm.

In view of the above, from another aspect, the present inventionprovides a distributed acoustic sensing system comprising a fibre opticcable wherein discrete acoustic coupling regions are interspersed alongthe length of the fibre optic cable.

As shown in the prior art, known distributed acoustic sensing systemsutilise clamps, which secure the cable to the structure or area that isbeing monitored. The clamps act as acoustic coupling points such thatthey detect acoustic signals by transmitting the acoustic energy of thesignals to the at least one optical fibre arrangement. In somesituations, the use of clamps is not possible and the fibre no longerconsists of an array of acoustic sensing points, resulting in aliasingeffects. To resolve this deficiency, the present invention provides afibre optic cable that includes the feature of discrete acousticcoupling points, wherein the acoustic coupling between the at least oneoptical fibre arrangement and the outer layer has been enhanced.Consequently, the cable may be deployed alongside a structure or area,without the use of clamps, and be used to detect acoustic signals.

Preferably, the locations of the discrete acoustic coupling regions areknown and match the resolution of a distributed acoustic sensor system.

Distributed acoustic sensor systems are able to resolve acoustic signalswith a spatial resolution of up to 1 m, thus it is preferable that theplurality of discrete acoustic sensors match this resolution. In doingthis, the sensing points will be phase matched, thus enhancing thedetection sensitivity.

A further aspect of the present invention provides a fibre optic cablewhich has an acoustic sensitivity that is non isotropic. This enablesincoming acoustic signals to be preferentially detected from particulardirections. Preferably, the acoustic sensitivity is adapted in at leastone or more directions extending in the plane normal to the length ofthe fibre optic cable.

In one embodiment of this aspect of the invention, a fibre optic cableis provided wherein a filler is inserted in a gap between at least oneoptical fibre arrangement and an outer layer, wherein the fillercomprises built up regions that bridge the gap between the at least oneoptical fibre arrangement and the outer layer so as to enable acousticenergy to be transmitted from the outer layer to the at least oneoptical fibre arrangement. Furthermore, the built up regions of fillerextend in at least one direction in the plane normal to the length ofthe fibre optic cable so as to directionally adapt the regions ofacoustic coupling.

A preferred embodiment of this aspect of the invention provides a fibreoptic cable wherein an acoustically reactive mass surrounds the fibreoptic cable. The acoustically reactive mass is preferably a materialwith high acoustic coupling such that it effectively transmits acousticenergy. This allows the acoustic sensitivity to be directionally adaptedin the plane normal to the length of the fibre optic cable, whereby thedirection of detection of incoming acoustic signals in this plane may bechosen.

Preferably, at least one segment of acoustic insulation is placed in theacoustically reactive mass so as to reduce the acoustic coupling of thefibre optic cable in at least one direction in the plane normal to thelength of the fibre optic cable. In doing this, acoustic signals will bedetected with greater sensitivity in the directions corresponding toregions of the mass where acoustic insulation has not been incorporated,thus adapting the acoustic sensitivity in these regions. For example,the acoustic insulation attenuates incoming acoustic waves fromdirections which are incident on the insulation, thus preventing orreducing the detection.

In another embodiment, at least one segment of acoustically reactivematerial is placed in the acoustically reactive mass surrounding thefibre optic cable so as to further adapt the acoustic sensitivity in atleast one direction in the plane normal to the fibre optic cable. Thiswill result in greater acoustic sensitivity in directions correspondingto regions where the segments of acoustically reactive material havebeen places such that incoming acoustic signals will be easily detected.

In one embodiment, the at least one segment of acoustically reactivematerial is arranged to be interspersed at intervals along the length ofthe fibre optic cable to produce a plurality of discrete acousticcoupling regions, which also have directional acoustic sensitivity.Alternatively, the at least one segment of acoustically reactive massmay extend substantially continuously along the length of the fibreoptic cable.

Another embodiment of this aspect of the invention provides a fibreoptic cable wherein an acoustically insulating mass surrounds the fibreoptic cable, whereby the acoustic sensitivity is directionally adaptedin the plane normal to the length of the fibre optic cable by theacoustically insulating mass. In doing this, acoustic signals will bedetected with lower sensitivity in directions corresponding to regionsof greater acoustic insulation.

Preferably, the fibre optic cable is placed in the acousticallyinsulting mass in a non-isotropic configuration so as to vary theacoustic coupling in the plane normal to the length of the fibre opticcable. That is to say, acoustic signals will be detected with greatersensitivity in regions corresponding to a higher acoustic coupling.

In another preferred embodiment of this aspect of the invention, theacoustically insulating mass has a non-isotropic configuration so as tovary the acoustic coupling in the plane normal to length of the fibreoptic cable. In doing this, the acoustic sensitivity is greater inregions wherein incoming acoustic waves are attenuated to a lesserdegree.

In one embodiment, at least one segment of acoustic insulation is placedin the acoustically insulating mass so as to further adapt the acousticsensitivity of the fibre optic cable in at least one direction in theplane normal to the length of the fibre optic cable. By doing this,acoustic signals will be detected with less sensitivity in thedirections corresponding to regions of the mass where acousticinsulation has been incorporated, thus adapting the acoustic sensitivityin these regions

In another embodiment, at least one segment of acoustically reactivematerial is placed in the acoustically insulating mass so as to furtheradapt the acoustic sensitivity in at least one direction in the planenormal to the fibre optic cable. This will result in greater acousticsensitivity in directions corresponding to regions where the segments ofacoustically reactive material have been places such that incomingacoustic signals will be easily detected.

In a further embodiment of the present invention, the at least onesegment of acoustically reactive material is arranged to be interspersedat intervals along the length of the fibre optic cable to produce aplurality of discrete acoustic coupling regions, which also havedirectional acoustic sensitivity. Alternatively, the at least onesegment of acoustically reactive material may extend substantiallycontinuously along the length of the fibre optic cable.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described by way of example only, andwith reference to the accompanying drawings in which:

FIG. 1 illustrates an example fibre optic cable of the prior art;

FIG. 2 illustrates a typical fibre optic cable deployment fordistributed acoustic sensing of the prior art;

FIG. 3 shows in schematic form a preliminary step in an embodiment ofthe present invention, whereby a layer of acoustic insulating materialis placed between the outer layer and the at least one optical fibre;

FIG. 4 shows in schematic form a first embodiment of the presentinvention, wherein acoustic sensing points are achieved by the insertionof a filler so as to produce regions of acoustic coupling;

FIG. 5 shows in schematic form a second embodiment of the presentinvention, whereby acoustic sensing points are achieved by crimping thefibre optic cable, thereby creating regions of acoustic coupling;

FIG. 6 shows in schematic form a further embodiment of the presentinvention, wherein a plurality of acoustic sensing points with differentperiodicities are provided within the same fibre optic cable;

FIG. 7 shows in schematic form the application of a fibre optic cableaccording to a embodiments of the present invention in systems ofdistributed acoustic sensing;

FIG. 8 shows a cross sectional view of another embodiment of the presentinvention whereby acoustic sensing points are produced so as to bedirectional in the plane normal to the length of the fibre optic cable;

FIG. 9 shows a graphical representation of the non isotropic acousticsensitivity in relation to the embodiment shown in FIG. 6;

FIG. 10 illustrates a preferred embodiment of the present invention,whereby regions of acoustic insulation are adapted so as to focusacoustic signals directionally in the plane normal to the length of thefibre optic cable;

FIG. 11 shows a graphical representation of the non isotropic acousticsensitivity in relation to the embodiment shown in FIG. 10;

FIG. 12 shows in schematic form another embodiment of the presentinvention, wherein a fibre optic cable is surrounded by a layer ofacoustic insulation material so as to provide non isotropic acousticsensitivity;

FIG. 13 shows a graphical representation of the non isotropic acousticsensitivity in relation to the embodiment shown in FIG. 12;

FIG. 14 shows in schematic form another embodiment of the presentinvention, wherein a fibre optic cable is surrounded by a layer ofacoustic insulation material so as to provide non isotropic acousticsensitivity;

FIG. 15 shows a graphical representation of the non isotropic acousticsensitivity in relation to the embodiment shown in FIG. 14;

FIG. 16 shows in schematic form another embodiment of the presentinvention, wherein a fibre optic cable is surrounded by a layer ofacoustic insulation material so as to provide non isotropic acousticsensitivity; and

FIG. 17 shows a graphical representation of the non isotropic acousticsensitivity in relation to the embodiment shown in FIG. 16;

FIG. 18 shows in schematic form another embodiment of the presentinvention, wherein a fibre optic cable is surrounded by a masscomprising portions of acoustic insulation and portions of material withhigh acoustic coupling;

FIG. 19 shows a graphical representation of the non isotropic acousticsensitivity in relation to the embodiment shown in FIG. 18.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In a particular embodiment of the invention, described here in order toprovide an example of a preferred implementation of the presentinvention, a distributed acoustic sensor is provided along a fibre opticcable, which emulates having a plurality of discrete sensing points. Inorder to emulate the discrete points of acoustic coupling, the acousticcoupling between the outer layer and the at least one optical fibrearrangement is adapted as will be described.

With reference to FIG. 3, there is provided a length of fibre opticcable 10 comprising at least one optical fibre arrangement 100surrounded concentrically by an outer layer 101, wherein a gap 200 isprovided between the at least one optical fibre arrangement 100 and theouter layer 101. The gap 200 comprises at least one acoustic insulatingmaterial, typically air, which exhibits low acoustic coupling. The airlayer 200 acts as a sound insulating layer between the outer layer 101and the at least one optical fibre arrangement 100 at least one opticalfibre arrangement 100.

A preferred embodiment of the present invention is illustrated by FIG.4, wherein a filler 102 is inserted into the gap 200 between the atleast one optical fibre arrangement 100 and the outer layer 101. Thefiller 102 is configured so as to provide built up regions at pointsinterspersed along the length of the fibre optic cable 20, thus creatingregions of acoustic insulation 201 between the built up regions offiller 102. The built up regions of filler 102 bridge the gap betweenthe outer layer 101 and the at least one optical fibre arrangement 100so as to produce regions of relative acoustic coupling 202. This couplesthe outer layer 101 and the at least one optical fibre arrangement 100such that the acoustic energy, as a result of acoustic signals incidenton the fibre optic cable 20, may be transmitted to the at least oneoptical fibre arrangement 100 at the acoustically coupling regions 202,hence enabling incident acoustic signals to be detected by the fibreoptic cable 20 at these points along its length. The regions of acousticinsulation 201 adapt the acoustic coupling between the at least oneoptical fibre arrangement 100 and the outer layer 101 such that thesepoints along the fibre optic cable 20 have lower acoustic coupling andthe transmission of acoustic energy is impeded, hence enhancing theeffect of the regions of acoustic coupling 202.

Preferably, the acoustic insulating regions 201 are typically 1-5 metresin length, so that the sensing resolution of the fibre optic cable 20matches the actual resolution of the points at which acoustic energy isbeing sensed. The built up regions of filler 102, that is to say thecoupling regions, are sufficiently small that they provide discretepoints at which the acoustic signal may be detected. For example, theacoustic coupling regions may be approximately 10 to 50 cm in length.The built up regions of filler 102 should not, however, be so small, forexample, smaller than 1 cm, that they do not provide a region largeenough to transmit the acoustic energy.

An alternative embodiment of the present invention is illustrated byFIG. 5, wherein a fibre optic cable 10 described by FIG. 3 is crimped,for example by a manual means, at intervals along its length to producefibre optic cable 30. The crimped portions 103 of the fibre optic cable30 are such that the inner face of the outer layer 101 comes intocontact with the at least one optical fibre arrangement 100, thereforebridging the insulating gap 200 between the outer layer 101 and the atleast one optical fibre arrangement 100 at that point. The fibre opticcable 30 is not crimped insofar that it squashes the at least oneoptical fibre arrangement 100 in any way. The crimped portions 103 thusprovide regions of acoustic coupling such that the outer layer 101 isable to transmit acoustic energy to the at least one optical fibrearrangement 100 at discrete points corresponding to the crimpedportions. This results in regions of acoustic insulation 201 along thelength of the fibre optic cable 30 between each point of acousticcoupling 103.

The crimped portions 103 couple the outer layer 101 and the at least oneoptical fibre arrangement 100 such that the acoustic energy, as a resultof acoustic signals incident on the fibre optic cable 30, may betransmitted to the at least one optical fibre arrangement 100, henceenabling acoustic signals to be detected by the fibre optic cable 30 atthese points along its length. The regions of acoustic insulation 201help to adapt the acoustic coupling between the at least one opticalfibre arrangement 100 and the outer layer 101 such that these pointsalong the fibre optic cable 30 have lower acoustic coupling and thetransmission of acoustic energy is impeded, hence enhancing the effectof the regions of acoustic coupling.

Preferably, the acoustic insulation regions 201 are typically 1-5 metresin length, so that the sensing resolution of the fibre optic cable 30matches the actual resolution of the points at which acoustic energy isbeing sensed, when, for example, the fibre optic cable 1 is held byclamps such as shown in FIG. 2. The crimped portions 103, that is to saythe coupling regions, are sufficiently small that they provide discretepoints at which the acoustic signal may be detected. Preferably, theacoustic coupling regions are 10 to 50 cm in length. The crimpedportions 103 should not, however, be so small, for example, smaller than1 cm, that they do not provide a region large enough to transmit theacoustic energy.

A further embodiment of the present invention is illustrated by FIG. 6,wherein a filler 203 is inserted into a gap 201 between at least oneoptical fibre arrangement 100 at least one optical fibre arrangement 100(for example, a FIMT) and an outer layer 101. Similar to thatillustrated by FIG. 4, the filler 203 is configured to produce aplurality of built up regions 204, 205 at points interspersed along thefibre optic cable 40 so as to provide discrete coupling points. Thebuilt up regions may then be periodically divided into smaller sections,thus producing smaller sensing points within each built up region thatare evenly spaced apart. For example, a first built up region 204 and asecond built up region 205 both represent discrete coupling points ofequal length along the fibre optic cable 40. The first built up region204 has been equally divided into two smaller coupling points 204 a-b,whereas the second built up region 205 has been equally divided intothree smaller coupling points 205 a-c. Preferably, the built up regionsare 10 to 50 cm in length along the fibre optic cable 40, and areperiodically divided such that the smaller coupling points areapproximately 1 to 5 cm in length along the fibre optic cable 40.

By periodically dividing the discrete coupling regions 204, 205, theresolution at which acoustic energy is sensed is increased since theperiodic structure of the discrete coupling regions 204, 205 increasesthe spatial resolution of the fibre optic cable 40.

Additionally, the periodic structure of the discrete coupling regions204, 205 can be used to track the eddy flow of a fluid contained withina pipeline or vessel being monitored by the fibre optic cable 40. Aneddy is a current of fluid that results when a fluid flows past anobject in its path, causing the current of the fluid to change directionwith respect to the general motion of the whole fluid. The individualeddies are capable of producing acoustic vibrations, and so by trackingthe eddies within the discrete regions of acoustic coupling, 204, 205,an object or defect in the vessel containing the fluid can be detected.To track the eddies, the periodic structure of the discrete couplingregions 204, 205 can be configured such that the spacing between theperiodic sending points 204 a-b, 205 a-c matches the life of the eddieswithin the monitored pipeline or vessel.

Another embodiment of the present invention is illustrated by FIG. 7,wherein a fibre optic cable 20 according to the present invention isused in conjunction with a system 50 for performing distributed acousticsensing (DAS), for example, the iDAS™, available from Silixa Limited, ofElstree, UK. In FIG. 7, a fibre optic cable 20 as described by FIG. 4 isshown, but it should be appreciated that any fibre optic cable accordingto the present invention may be used in DAS systems. The DAS system 50is capable of obtaining a measurement profile along the length of thefibre optic cable 20, digitally recording acoustic fields at intervalsalong at least one optical fibre 51 contained within the fibre opticcable 20.

A DAS system 50 injects pulsed light into the at least one optical fibre51 which propagates down the entire length of the at least one opticalfibre 51. Light that is then reflected or back scattered by the at leastone optical fibre 51 is returned to the DAS system 50, wherein theoptical phase data of the returned signal is measured, such thatvariations in the optical path of the returned signal due to acousticvibrations are detected. Preferably, the optical phase data measurementsare made at discrete sampling points along the length of the at leastone optical fibre 51 so that the position of any acoustic vibrations maybe determined.

In FIG. 7, the DAS system 50 is controlled such that it is possible toposition where the DAS system 50 takes its measurements along the lengthof the at least one optical fibre 51, by time synchronising the pulsedlight with the locations of the discrete coupling regions 202. Forexample, the DAS system 50 can control its internal processing such thatthe positions of its effective acoustic measurement points can becontrolled with respect to the positions of the discrete couplingregions. In this respect, the DAS system 50 measures the optical phasedata of any light reflected or back scattered 52 a-b from along thefibre, with changes in the back scatter as a result of incident acousticvibrations being detected and used to recreate the incident acousticsignal. The processing performed in the DAS can be controlled such thatthe effective acoustic measurement points along the fibre can be setwith respect to the positions of the discrete acoustic coupling regions.For example, as described above in many embodiments it will bebeneficial to control the positions of the acoustic measurement pointsalong the fibre so as to coincide with the positions of the discreteacoustic coupling regions. However, in other embodiments there may bemodes of operation, such as test modes or calibration modes, or evensome operational modes, where it is desirable to synthetically shift (asa result of the signal processing applied in the DAS) the acousticmeasurement points with respect to the acoustic coupling regions.

For example, in a test or calibration mode it may be desirable to “move”the acoustic sampling points to be between the acoustic couplingregions, such that acoustic coupling to the sensing points is minimised,so as to reduce background noise for testing or calibration purposes.Additionally or alternatively, in some operational scenarios it may bedesirable to synthetically “move” the acoustic sampling points away fromthe acoustic coupling regions, if for example the acoustic couplingregions are enhancing or highlighting one signal (for example viaresonant effects) to the detriment of the detection of others. It willtherefore be understood that the DAS can control the relative positionsof acoustic sampling points along the fibre with respect to thepositions of the acoustic coupling regions, so as to make them coincide,or to be displaced from each other by a varying controllable amount. Forexample, the acoustic sampling points can be controlled so as topositionally coincide with the acoustic coupling regions (e.g. be inphase with each other), which is the envisaged preferred mode ofoperation for most applications, or controlled so as to be in anyposition between the acoustic sampling regions, including, to give aminima signal, positioned substantially half-way between the acousticcoupling regions i.e. such that the acoustic coupling regions and theacoustic sampling points along the fibre are essentially located inanti-phase positions with respect to each other.

In a second aspect of the present invention, a fibre optic cable isprovided wherein the acoustic sensitivity of the cable is non isotropic.The fibre optic cable is adapted to provide regions of directionalacoustic coupling such that incident acoustic signals are only detectedfrom particular directions. Examples of how this non isotropicsensitivity may be achieved is described below.

A further embodiment, with reference to FIG. 8, provides a fibre opticcable 60 comprising a at least one optical fibre arrangement 100surrounded concentrically by an outer layer 101, such that a gap 200 isprovided between the at least one optical fibre arrangement 100 and theouter layer 101. A filler 104 is inserted between the at least oneoptical fibre arrangement 100 and the outer layer 101, and is configuredto provide built up regions which bridge the gap between the outer layer101 and the at least one optical fibre arrangement 100 in order tofacilitate the transfer of acoustic vibrations to the at least oneoptical fibre arrangement 100. The built up regions are located in atleast one direction in the plane normal to the length of the fibre opticcable 60, so as to provide acoustic coupling that is directional in theplane normal to the fibre optic cable 60.

This results in regions of acoustic insulation 201, with low acousticcoupling, in all other directions in the plane normal to the length ofthe fibre optic cable 60 other than the built up regions. That is to saythat incoming acoustic signals will be detected with greater sensitivityin the direction corresponding to the acoustic coupling points producedby the built up regions of filler 104. Additionally, in some embodimentsthe built up regions may also be interspersed along the length of thefibre optic cable 60, as illustrated by the embodiment shown in FIG. 4,so as to provide discrete coupling points along the length of the fibreoptic cable 60 that are also directional in the plane normal to thefibre optic cable 60. In other embodiments, however, the filler regions104 extend substantially continuously along the length of the fibreoptic cable 60.

FIG. 9, by way of example, illustrates a possible distribution ofacoustic sensitivity 61 and 62 that results from a fibre optic cable 60,as shown in FIG. 8. FIG. 9 shows a response plot relating to the fibreoptic cable 60 in the plane normal to its length and the resultingacoustic sensitivity distribution 61 and 62 corresponding to theconfiguration of fibre optic cable 60 given in FIG. 8. The acousticsensitivity 61 is focused and enhanced by the regions of acousticcoupling produced by the built up regions of filler 104, since these arethe most acoustically reactive regions of the fibre optic cable 60. Incomparison, the acoustic sensitivity 62 corresponding to the regions ofacoustic insulation 201 is reduced. As a result, an incident acousticsignal is detected by these coupling regions more readily, resulting inan acoustic sensitivity profile 60 and 62, as shown, which is not onlydirectional but also dependent on the size of the regions of acousticcoupling.

A further alternative embodiment is shown in FIG. 10, whereby a fibreoptic cable 1 is surrounded by an acoustically reactive mass 300.Segments of acoustic insulation 301 are inserted into the mass 300 so asto provide regions of low acoustic coupling in specific directions inthe plane normal to the length of the fibre optic cable 1. Theinsulation results in regions of higher acoustic coupling between thesegments of acoustic insulation 301, such that acoustic signals are morereadily detected in the directions of the plane normal to the length ofthe fibre optic cable 1 corresponding to these regions of higheracoustic coupling.

FIG. 11, by way of example, illustrates a possible distribution ofacoustic sensitivity 302 and 303 that results from a fibre optic cable 1surrounded by an acoustically reactive mass 300, such as that shown inFIG. 10. FIG. 11 shows the fibre optic cable 1 in the plane normal toits length and the acoustic sensitivity distribution 302 and 303corresponding to the configuration provided by the embodiment of FIG.10. The acoustic sensitivity 302 is focused and enhanced by the regionsof higher acoustic coupling that result from the segments of acousticinsulation 301 inserted into the acoustically reactive mass 300 into thenodal regions. The nodal regions of higher acoustic coupling detectincoming acoustic signals more readily, resulting in regions of acousticsensitivity 302 which correspond to the position and size of the regionsof higher acoustic coupling. In comparison, the acoustic sensitivity 303corresponding to the segments of insulation acoustic 301 is reduced.

Another further embodiment is illustrated in FIG. 12, wherein a fibreoptic cable 1 is surrounded by a layer of acoustic insulation material400, but such that the fibre optic cable 1 is not positioned centrallywithin the acoustic insulation 400. The acoustic insulation material 400has low acoustic coupling properties and impedes incoming acousticsignals. The fibre optic cable 1 is located within the acousticinsulation 400 such that the distance from the outer edge of the fibreoptic cable 1 to the outer edge of the acoustic insulation 400 variesaround the circumference of the fibre optic cable 1. The region wherethis distance is smallest has higher acoustic sensitivity since incomingacoustic signals are impeded to a lesser degree.

FIG. 13 illustrates a possible resulting acoustic sensitivity profile401 and 402 for a fibre optic cable 1 surrounded by acoustic insulationmaterial 400, such as that shown in FIG. 12. FIG. 13 shows the fibreoptic cable 1 in the plane normal to its length and the distribution ofacoustic sensitivity 401 and 402 corresponding to the embodiment of FIG.12. The acoustic sensitivity 401 is focused and enhanced by the regionwherein the distance from the outer edge of the fibre optic cable 1 tothe outer edge of the acoustic insulation 400 is smallest since this isthe region where incoming acoustic signals are least impeded and morereadily detected. In comparison, the acoustic sensitivity 402,corresponding to regions wherein the distance from the outer edge of thefibre optic cable 1 to the outer edge of the acoustic insulation 400 islargest, is reduced.

A further embodiment is illustrated by FIG. 14, wherein a fibre opticcable 1 is surrounded by a layer of acoustic insulation material 500,wherein the acoustic insulation 500 is of square configuration. Theacoustic insulation material 500 has low acoustic coupling propertiesand impedes incoming acoustic signals. The fibre optic cable 1 islocated centrally within the acoustic insulation 500, but due to theshape of the acoustic insulation 500, the distance from the outer edgeof the fibre optic cable 1 to the outer edge of the acoustic insulation500 varies around the circumference of the fibre optic cable 1. Regionswhere this distance is smaller have higher acoustic sensitivity sinceincoming acoustic signals are impeded to a lesser degree.

FIG. 15 illustrates a possible resulting acoustic sensitivity profile501 for a fibre optic cable 1 surrounded by acoustic insulation material500, such as that shown in FIG. 14. FIG. 15 shows the fibre optic cable1 in the plane normal to its length and the distribution of acousticsensitivity 501 corresponding to the embodiment of FIG. 14. The acousticsensitivity 501 is focused and enhanced by the regions wherein thedistance from the outer edge of the fibre optic cable 1 to the outeredge of the acoustic insulation 500 is smaller since these are theregions where incoming acoustic signals are least impeded and morereadily detected. In comparison, the acoustic sensitivity 501corresponding to regions wherein the distance from the outer edge of thefibre optic cable 1 to the outer edge of the acoustic insulation 500 islargest, is reduced.

A further embodiment is illustrated by FIG. 16, wherein a fibre opticcable 1 is surrounded by a layer of acoustic insulation material 600,wherein the acoustic insulation 600 is of star configuration. Theacoustic insulation material 600 has low acoustic coupling propertiesand impedes incoming acoustic signals. The fibre optic cable 1 islocated centrally within the acoustic insulation 600, but due to theshape of the acoustic insulation 600, the distance from the outer edgeof the fibre optic cable 1 to the outer edge of the acoustic insulation600 varies around the circumference of the fibre optic cable 1. Regionswhere this distance is smaller have higher acoustic sensitivity sinceincoming acoustic signals are impeded to a lesser degree.

FIG. 17 illustrates a possible resulting acoustic sensitivity profile601 for a fibre optic cable 1 surrounded by acoustic insulation material600, such as that shown in FIG. 16. FIG. 17 shows the fibre optic cable1 in the plane normal to its length and the distribution of acousticsensitivity 601 corresponding to the embodiment of FIG. 16. The acousticsensitivity 601 is focused and enhanced by the regions wherein thedistance from the outer edge of the fibre optic cable 1 to the outeredge of the acoustic insulation 600 is smaller since these are theregions where incoming acoustic signals are least impeded and morereadily detected. In comparison, the acoustic sensitivity 601,corresponding to regions wherein the distance from the outer edge of thefibre optic cable 1 to the outer edge of the acoustic insulation 600 islargest, is reduced.

Another preferred embodiment is illustrated by FIG. 18, wherein a fibreoptic cable 1 is concentrically surrounded by a casing 700 which may bemade from an acoustically insulating material that impedes themechanical vibrations of any acoustic signals which are incident on it.Alternatively, the casing 700 may be made from an acoustically reactivematerial that easily couples with the mechanical vibrations of anyacoustic signals that are incident on it. The casing 700 may comprise aplurality of acoustically reactive segments 701 a-b made of a materialwith high acoustic coupling properties so as to directionally enhancethe acoustic sensitivity in the plane normal to the length of the fibreoptic cable 1. In regions that include acoustically reactive segments701 a-b, the fibre optic cable 1 will more easily couple to acousticvibrations and, therefore, more readily detect incoming acousticsignals. The acoustically reactive segments 701 a-b may be made of anumber of different materials with high acoustic coupling properties,for example, a metal, and may be positioned anywhere within the sleeve700.

Additionally, the casing 700 may comprise a plurality of segments ofacoustic insulation 702 a-b with low acoustic coupling properties sothat incident acoustic signals are impeded in these regions. That is tosay, the acoustic sensitivity in directions of the plane normal to thelength of the fibre optic cable 1 will be reduced as a result of thesegments of acoustic insulation 702 a-b. The segments of acousticinsulation 702 a-b may be made of a number of materials with lowacoustic coupling properties that act as good acoustic insulators, forexample, air, and may be positioned anywhere within the sleeve. In someembodiments, the acoustically reactive segments 701 a-b may also beinterspersed along the length of the fibre optic cable 1, as illustratedby the embodiment shown in FIG. 4, so as to provide discrete acousticcoupling points along the length of the fibre optic cable 1 that arealso directional in the plane normal to the fibre optic cable 1. Inother embodiments, however, the acoustically reactive segments 701 a-bextend substantially continuously along the length of the fibre opticcable 1.

FIG. 19 illustrates a possible resulting acoustic sensitivity profile703, 704 for a fibre optic cable 1 surrounded by a sleeve 700 thatincludes acoustically reactive segments 701 a-b and portions of acousticinsulation 702 a-b, such as that shown in FIG. 18. FIG. 19 shows aresponse plot relating to the fibre optic cable 1 in the plane normal toits length, and shows the acoustic sensitivity distribution 703, 704corresponding to the configuration of the sleeve 700 surrounding thefibre optic cable 1 shown in FIG. 18. The acoustic sensitivity 703 isfocused and enhanced by the regions corresponding to the acousticallyreactive segments 701 a-b since these are the most acoustically reactiveregions of the arrangement. In comparison, the acoustic sensitivity 704corresponding to the portions of acoustic insulation 702 a-b arereduced. Therefore, any incident acoustic signals are more readilydetected by the regions of higher acoustic coupling 701 a-b, resultingin the acoustic sensitivity profile of FIG. 19, which is not onlydirectional but also dependent on the size of the acoustically reactivesegments 701 a-b and the portions of acoustic insulation 702 a-b.

FIGS. 9, 11, 13, 15, 17 and 19 are to be taken as projecteddistributions of acoustic sensitivity for the respective fibre opticcable embodiments and are merely indicative for qualitativeunderstanding purposes only. The actual resulting acoustic sensitivitiesmay differ from these embodiments and these profiles are only intendedto provide an indication of their appearance.

Alternative embodiments may include fibre optic cables 1 with at leastone optical fibre arrangement 101 that is not comprised of a FIMT asdescribed by the prior art FIG. 1, but of some other arrangementcomprising at least one optical fibre encapsulated in a sealed tube.

Another example of a further embodiment may be a fibre optic cable 1,where the narrowed portions along the fibre optic cable 1 are achievedby some means other than crimping, such as an outer layer which ismanufactured so as to include regions which are curved inwards so as tomake contact with the at least one optical fibre arrangement 100 atpoints interspersed along the length of the fibre optic cable 1. Thesepoints of contact produce the regions of acoustic coupling.

Another further embodiment is a fibre optic cable 1 that uses anacoustic insulation material other than air, such as an acoustic foam.Additionally, a combination of acoustic insulators may be used in orderto provide regions of low acoustic coupling and hence enhance theacoustic coupling between the at least one optical fibre arrangement 100and outer layer 101. Preferably, the acoustic insulating materials arechosen such that the acoustic coupling of the acoustic insulating regionis of an optimum value to prevent the transmission of acoustics energyor at least sufficiently different to that of the discrete couplingregions so as to impede the acoustic signal at a different rate.

Optionally, an acoustically reactive mass 300, as shown in FIG. 10, maybe located at points interspersed along the length of the fibre opticcable 1 so as to produced discrete coupling points along the length ofthe fibre optic cable 1, which are also directional in the plane normalto the length of the fibre optic cable 1.

A further modification may include a fibre optic cable, surrounded by alayer of acoustic insulation wherein the distance from the outer edge ofthe fibre optic cable to the outer edge of the acoustic insulation isnot uniform around the circumference of the fibre optic cable, thusresulting in an acoustic sensitivity profile that varies directionallyand is non isotropic.

Various modifications, whether by way of addition, deletion orsubstitution may be made to the above described embodiments to providefurther embodiments, any and all of which are intended to be encompassedby the appended claims.

1. A fibre optic cable, wherein the fibre optic cable has an acousticsensitivity that is non isotropic.
 2. A fibre optic cable according toclaim 1, wherein the acoustic sensitivity is adapted in at least one ormore directions extending in a plane normal to the length of the fibreoptic cable.
 3. A fibre optic cable according to claim 1, wherein anacoustically reactive mass surrounds the fibre optic cable; wherebyacoustic sensitivity is directionally adapted in a plane normal to thelength of the fibre optic cable by the reactive mass.
 4. A fibre opticcable according to claim 3, wherein at least one segment of acousticinsulation is placed in the acoustically reactive mass so as to furtheradapt the acoustic sensitivity of the fibre optic cable in at least onedirection in the plane normal to the length of the fibre optic cable. 5.A fibre optic cable according to claim 3, wherein at least one segmentof acoustically reactive material is placed in the acoustically reactivemass surrounding the fibre optic cable so as to further adapt theacoustic sensitivity in at least one direction in the plane normal tothe fibre optic cable.
 6. A fibre optic cable according to claim 5,wherein the at least one segment of acoustically reactive material isarranged to: a) be interspersed at intervals along the length of thefibre optic cable to produce a plurality of discrete acoustic couplingregions; or b) extend substantially continuously along the length of thefibre optic cable.
 7. A fibre optic cable according to claim 1, whereinan acoustically insulating mass surrounds the fibre optic cable; wherebyacoustic sensitivity is directionally adapted in a plane normal to thelength of the fibre optic cable by the acoustically insulating mass. 8.A fibre optic cable according to claim 7, wherein the fibre optic cableis located in the acoustically insulating mass in a non-isotropicconfiguration so as to vary the acoustic sensitivity in the plane normalto the length of the fibre optic cable.
 9. A fibre optic cable accordingto claim 8, wherein the acoustically insulating mass has a non-isotropicconfiguration so as to vary the acoustic impedance in the plane normalto the length of the fibre optic cable.
 10. A fibre optic cableaccording to claim 7, wherein at least one segment of acousticinsulation is placed in the acoustically insulating mass so as tofurther adapt the acoustic sensitivity of the fibre optic cable in atleast one direction in the plane normal to the length of the fibre opticcable.
 11. A fibre optic cable according to claim 7, wherein at leastone segment of acoustically reactive material is placed in theacoustically insulating mass so as to further adapt the acousticsensitivity in at least one direction in the plane normal to the fibreoptic cable.
 12. A fibre optic cable according to claim 11, wherein theat least one segment of acoustically reactive material is arranged to:a) be interspersed at intervals along the length of the fibre opticcable to produce a plurality of discrete acoustic coupling regions; orb) extend substantially continuously along the length of the fibre opticcable.
 13. A fibre optic cable according to claim 1, further comprisingan acoustic insulating layer between at least one optical fibrearrangement and an outer layer, wherein discrete acoustic couplingregions are interspersed along the length of the fibre optic cable fortransmitting acoustic energy from the outer layer to the at least oneoptical fibre.
 14. A fibre optic cable according to claim 13, whereinthe acoustic insulating layer includes a layer of air.
 15. A fibre opticcable according to claim 13, wherein at least one layer concentricallyoutside the acoustic insulating layer is narrowed at points interspersedalong the length of the fibre optic cable so as to divide the acousticinsulating layer and provide the discrete acoustic coupling regions. 16.A fibre optic cable according to claim 15, wherein the fibre optic cableis crimped at points interspersed along its length such that an innerface of the outer layer comes into contact with the at least one opticalfibre; wherein the crimped points provide discrete acoustic couplingregions.
 17. A fibre optic cable according to claim 16, wherein adistance between the acoustic coupling regions is at least 1 m.
 18. Afibre optic cable according to claim 13, wherein the discrete acousticcoupling regions comprise a periodic structure.
 19. A fibre optic cableaccording to claim 18, wherein the periodic structure is achieved bydividing the discrete acoustic coupling regions into equal portions. 20.A distributed acoustic sensing system, comprising a fibre optic cable ofclaim 1.